a) Field of the Invention
The invention relates to a direct type backlight device for use as a backlight source of a liquid crystal display device.
b) Description of Related Art
Liquid crystal display (LCD) devices used as display apparatuses for microcomputers or television receivers do not emit light themselves, and therefore LCD devices must have a light source such as a lighting device.
Such lighting device commonly uses small diameter fluorescent tubes like cold-cathode tubes or hot-cathode tubes, and can be generally categorized into edge light type or direct type. In the edge light type, fluorescent tubes are placed at the side of a light-guide plate such that light emits from the LCD side of the light-guide plate. In the direct type, a reflecting plate and at least one fluorescent tube are housed inside a frame, wherein the direct light from the fluorescent tube and the reflected light that is the light from the fluorescent tube reflected by the reflecting plate are made incident into a diffusing plate placed at an opening of the frame, and then the light is diffused and emitted as an uniform planar light, forming an illuminated area.
Moreover, to evenly distribute the luminance of the illuminated area, the fluorescent tubes in the direct type lighting device are arranged in parallel with almost equal spacing, and so by arranging the increasing number of fluorescent tubes accordingly, the luminance distribution will not be degraded and the required luminance level can be obtained easily.
As display devices such as monitors and television receivers become larger in size and have higher luminance level, the increase in the number of fluorescent tubes used and the cost start to be problematic. To lower the cost, a method that uses one fluorescent tube to achieve the same luminance level and luminance distribution produced by multiple fluorescent tubes was developed by bending a long fluorescent tube and arranging it in parallel.
FIGS. 5A to 5E show, respectively, the front view, cross-sectional view, side views and bottom view of a conventional direct type lighting device, wherein the conventional lighting device includes: a frame 1; at least one fluorescent tube 2; a diffusing plate 3; an illuminated area 4; a frontal frame 5 for securing the diffusing plate 3 and preventing light leakage from surroundings; a reflecting plate 6 for effectively reflecting light from the fluorescent tube 2; a tube-holding frame 7 for holding the fluorescent tube 2; and a separation plate 8 for keeping a constant distance between the diffusing plate 3 and the reflecting plate 6.
The fluorescent tube 2 is bent at its center section to make at least two portions of the fluorescent tube 2 parallel, and the fluorescent tube 2 is arranged such that the width between the parallel parts of the fluorescent tube 2 is about the same as the distance between one parallel part and adjacent fluorescent tube 2.
However, the light-emitting length of a fluorescent tube per unit of illuminated area at the bent portion is longer than the light-emitting length of a fluorescent tube at the non-bent portion in the aforementioned bent fluorescent tube, and thereby the luminance distribution of the illuminated area is poor.
The current improvement method is to place the bent portion of the fluorescent tube outside of the illuminated area.
Problems to be Solved by the Invention
Because the bent portion of the aforementioned bent fluorescent tube in the direct type lighting device is placed outside of the illuminated area, the lighting device cannot extensively utilize the light from the bent portion of the fluorescent tube. Therefore, how to reduce the consumption of electricity or improve the luminance level becomes a major issue.
For the lighting device to use the light from the fluorescent tube extensively, the bent portion must be arranged in display region, but as aforementioned, the luminance distribution in the illuminated area degrades because the light-emitting length of the fluorescent tube per unit of illuminated area at the bent portion is longer than the light-emitting length of the fluorescent tube at non-bent portion.